A vaccine can contain a whole variety of different antigens. Examples of antigens are whole-killed organisms such as inactivated viruses or bacteria, fungi, protozoa or even cancer cells. Antigens may also consist of subfractions of these organisms/tissues, of proteins, or, in their most simple form, of peptides. Antigens can also be recognized by the immune system in form of glycosylated proteins or peptides and may also be or contain polysaccharides or lipids. Short peptides can be used since for example cytotoxic T cells (CTL) recognize antigens in form of short usually 8–11 amino acids long peptides in conjunction with major histocompatibility complex (MHC) (Rammensee et al., 1995).
In order to obtain sustained, antigen-specific immune responses, adjuvants need to trigger immune cascades that involve all cells of the immune system necessary. Primarily, adjuvants are acting, but are not restricted in their mode of action, on so-called antigen presenting cells (APC), of which dendritic cells (DCs) are the most potent. These cells usually first encounter the antigen(s) followed by presentation of processed or unmodified antigen to immune effector cells. Intermediate cell types may also be involved. Only effector cells with the appropriate specificity are activated in a productive immune response. The adjuvants may also locally retain antigens and co-injected other factors. In addition, the adjuvants may act as a chemoattractant for other immune cells or may act locally and/or systemically as a stimulating agent for the immune system.
Cells of the innate immune system recognize patterns expressed on their respective targets. Examples are lipopolysaccharides (LPS) in the case of Gram-negative bacteria, mycobacterial glycolipids, lipoteichoic acids of Gram-positive bacteria, mannans of yeast and double stranded RNAs of viruses (Hoffmann et al., 1999). In addition, they may recognize patterns such as altered glycosylations of proteins on tumor cells.
Polycationic polymers, for example the polycationic amino acid polymers poly-L-arginine and poly-L-lysine, have been shown to allow very efficient charging of APC with antigens in vitro and in vivo (Buschle et al., 1998, Buschle et al., 1997, Schmidt et al., 1997). This is thought to be the key event for triggering immune cascades, eventually leading to the induction of antigen-specific immune effector cells that are able to destroy or neutralize targets. It has been shown previously that a number of polycationic compounds exert effects on immune cells (Buschle et al., 1998, Buschle et al., 1997).
Co-injection of a mixture of poly-L-arginine or poly-L-lysine together with an appropriate antigen as a vaccine protects animals from tumor growth in several animal models (Buschle et al., 1998, Schmidt et al., 1997). This chemically defined vaccine is able to induce a high number of antigen-specific T cells. In order to induce antigen-specific T cells, peptides need to be taken up by APC. Such peptide-loaded APC will induce an immune cascade, eventually leading to the induction of antigen-specific immune effector cells like T cells.
Polyinosinic-polycytidylic acid (poly I:C) is known as a potent interferon type I inducer (Manetti et al., 1995). Because of its protective effects in a number of animal species against a broad spectrum of both RNA and DNA viruses (e.g., herpes simplex virus, rabies virus, Japanese B encephalitis virus, vaccinia virus, encephalomyocarditis virus), poly I:C is often used in models of viral infections. Changes that occur in response to poly I:C are thought to be representative of changes that occur in response to a variety of different viruses. Poly I:C is known to stimulate macrophages to produce cytokines such as IL-la and IL-12 (Manetti et al., 1995), it is a potent NK cell stimulator (Cavanaugh et al., 1996) and, in general, this compound is known to promote Th1-specific immune responses. Because of these abilities, poly I:C has been widely applied as an immunomodulator in several clinical trials showing little or no toxicity (Guggenheim et al., 1977, Simnaler et al., 1977). However, there was no patient benefit. It is unclear whether poly I:C on its own has adjuvant activity. Recent findings show that poly I:C also induces stable maturation of in vitro-cultured DCs and that such DCs are potent T cell stimulators in vitro (Cella et al., 1999; Verdijk et al., 1999).